This invention relates to cathode ray tubes, and particularly to color picture tubes of the type useful in home television receivers, and to electron guns therefor.
As shown schematically in FIG. 1, electron guns typically used in color picture tubes comprise a plurality of aligned electrodes including a cathode 2, control grid 3, screen grid 4, and two or more focusing electrodes 5 and 6. That portion of the gun up to the screen grid constitutes the beam forming region 7 and that portion beyond the screen grid constitutes the focusing region 8. In the operation of these guns, electrons 9 are emitted from the cathode and converged to a crossover 10 in the vicinity of the screen grid. This crossover is then imaged at an image plane on a screen 11 as a small spot by a main focus lens established between electrodes 5 and 6 in the focusing region of the gun. The convergence angle .alpha. at which the electrons approach the crossover is herein termed the crossover entrance angle, and the divergence angle .beta. at which the electrons leave the crossover is herein termed the crossover exit angle. The angles .alpha. and .beta. would be substantially equal to each other in the absence of any deflection field at the crossover. However, in actual practice the presence of electric fields in this region causes a constant bending of the electron rays as they enter and exit from the crossover, thus producing a complex crossover and a difference in the angles .alpha. and .beta..
Most workers in the art have generally believed that there is little interplay between the beam forming region 7 and the focusing region 8 of the gun; and when attention has been given to one of these two regions for improving the electron gun, usually little note has been given to the other. Notwithstanding this belief in the prior art, we have found that the first crossover, which is imaged on the screen by the focusing system of the gun, is much further forward in the gun than where it was heretofore believed to be. This has in turn led us to realize the interdependence between this beam-forming function of the gun and the subsequent focusing function of the gun. As a result, we have discovered that a judicious choice and combination of design parameters of the gun can produce an unexpected improvement in beam-spot performance of the gun.